Dr. Robert L. Morrison, Jr.
MIT Lincoln Laboratory, Lexington, MA
Inverse Synthetic Apeture Radar (ISAR) is a powerful technology for space-object characterization, in which ground-based radars interrogate orbiting satellites with high-bandwidth pulses to produce fully resolved images. These images are critical to the space surveillance mission, providing analysts with rich feature data for generating timely assessments of target size, status, and functionality. The Haystack Ultra-wideband Satellite Imaging Radar (HUSIR) upgrade, completed by MIT Lincoln Laboratory in 2013, enables exquisite 2-cm-resolution ISAR imaging of satellites, thus advancing the frontier in small-target characterization and illustrating recent advances in the field. As imaging radars evolve to support higher frequencies and larger bandwidths, the image formation process becomes more challenging. Understanding and addressing the challenges will be critical in developing next-generation ISAR systems. This tutorial provides an overview of ISAR imaging of satellites, with focus on recently developed sensors and state-of-the-art algorithms. First, the fundamentals of ISAR image formation are presented. Processing steps and challenges associated with image formation are treated, including motion compensation (range alignment and autofocus), pulse equalization, and resampling. The problem of motion compensation becomes particularly stressing at high frequencies, where uncompensated target motion must be corrected to within a small fraction of a wavelength to prevent significant image artifacts. The latest data-driven algorithms designed specifically for this purpose are highlighted. Image enhancement approaches including bandwidth extrapolation are covered with illustrating examples. The second portion of the tutorial focuses on image interpretation. This includes a discussion of features that can be derived from radar images of satellites. Factors affecting image interpretation, such as geometry, target motion, and radar-specific phenomena (shadowing, extended returns, and speculars) are explored. The tutorial concludes with an overview of advanced topics in ISAR, including 3D interferometric ISAR imaging and image reconstruction from partial data.